I recently upgraded one of the Exchange Mailbox 2016 Servers with CU19 while the other one is still running on CU11. After upgrading the second mailbox server to CU19, first mailbox server (CU11) can't send mails to another mailbox server (CU19) and vice-versa. Getting "Exchange Auth Failure". When checked logs, getting "target machine actively refused the connection". Default receive connector is having "Exch. Server Auth." enabled. I am even able to TELNET both the mailbox servers from each other. No SMTP inspection enables on firewalls. Any ideas please?